1. Field of the Invention.
This invention relates to a process for manufacturing chlorine dioxide by reducing a chlorate in a strong acid in the presence of a catalyst.
2. Description of Prior Art
Chlorine dioxide is a commercially important material in such fields as pulp bleaching, water treatment and fat decoloring, and also has recently been used in the fields of denitration of industrial waste gases and removal of phenols from industrial sewage. Thus it is highly desirable to have a process by which chlorine dioxide can be manufactured economically. Further, it is desirable to have a safe process in which the generation of chlorine dioxide can be easily controlled without any danger of explosion.
One of the methods for generating chlorine dioxide is to reduce a chlorate with a reducing agent in a strong acid. The reactions which occur are exemplified below, wherein, for the sake of illustration, the chlorate used is sodium chlorate and the reducing agent is hydrochloric acid. EQU NaClO.sub.3 + 2HCL.fwdarw.ClO.sub.2 + 1/2Cl.sub.2 + NaCl + H.sub.2 O (1) EQU naCLO.sub.3 + 6HCL.fwdarw.3Cl.sub.2 + NaCl + 3H.sub.2 O (2)
chlorine dioxide is formed by reaction (1), but not formed by reaction (2) which competes with reaction (1).
Accordingly, reaction (1) must be accelerated in order to generate chlorine dioxide efficiently. A useful method to achieve this purpose is to use a catalyst which accelerates reaction (1) in preference to reaction (2).
Palladium was described in Japanese Patent Publication No. 2,645/1970 as a catalyst for chlorine dioxide production. Vanadium pentoxide, silver ion, manganese ion, dichromate ion and arsenic ion were described in U.S. Pat. No. 3,563,702 for the same usage. Further, for the same purpose Japanese Patent Publication Nos. 4,119/1960, 7,301/1962, 14,958/1964, 17,047/1966 disclose manganese compounds; silver ions or a combination of silver ion and manganese ion; manganese (II) chelate compounds alone or combination of manganese chelate compounds and metal sequestering agents; lead ion or a combination of lead ion, manganese ion and silver ion; respectively. It is well known in the field of this invention that the ratio of the rate of reaction (1) to that of reaction (2) decreases at lower values of the chlorate-to-reducing agent molar ratio and at lower acid concentrations of the reaction medium, although the generation of chlorine dioxide under such conditions can be controlled more easily. However, these catalysts do not have satisfactory activities under the conditions described above. Even with palladium which shows the highest activity among them, the ratio of the rate of reaction (1) to that of reaction (2) found under the following conditions scarcely exceeds the value of 17: an acid concentration of 0.4 moles per liter of the reaction medium, a chlorate-to-reducing agent molar ratio of 0.27 and a palladium (II) concentration of 0.001 mole per liter of the reaction medium.
Recently, a process for manufacturing chlorine dioxide at lower acid concentrations and at remarkably high concentrations of a reducing agent (e.g., British Pat. No. 1,347,740) has been developed. Under such reaction conditions even more active catalysts are required in order to generate chlorine dioxide without loss of efficiency.